Logger MDC auto-instrumentation

The Mapped Diagnostic Context (MDC) is

an instrument for distinguishing interleaved log output from different sources. 用于区分不同来源的交错日志输出的仪器。
— log4j MDC documentation

It contains thread-local contextual information which is later copied to each logging event captured
by a logging library.

它包含线程本地的上下文信息，这些信息后来被复制到由日志库捕获的每个日志事件中。

The OTel Java agent injects several pieces of information about the current span into each logging
event’s MDC copy:

• trace_id - the current trace id
  (same as Span.current().getSpanContext().getTraceId());
• span_id - the current span id
  (same as Span.current().getSpanContext().getSpanId());
• trace_flags - the current trace flags, formatted according to W3C traceflags format
  (same as Span.current().getSpanContext().getTraceFlags().asHex()).

Those three pieces of information can be included in log statements produced by the logging library
by specifying them in the pattern/format. Example for Spring Boot configuration (which uses
logback):

logging.pattern.console = %d{yyyy-MM-dd HH:mm:ss} - %logger{36} - %msg trace_id=%X{trace_id} span_id=%X{span_id} trace_flags=%X{trace_flags} %n

This way any services or tools that parse the application logs can correlate traces/spans with log
statements.

Supported logging libraries